The United States on Thursday refused to recognize Venezuelan President Nicolas Maduro's legitimacy as he started a controversial new term and urged rank-and-file government employees to empower the opposition.

"The US will not recognize the Maduro dictatorship's illegitimate inauguration," national security advisor John Bolton tweeted.

"We will continue to increase pressure on the corrupt regime, support the democratic National Assembly, and call for democracy and freedom in Venezuela," he wrote.

Secretary of State Mike Pompeo called on Venezuelans to work not with Maduro but with the National Assembly, which is controlled by the opposition but has been sidelined by a new Constituent Assembly created by Maduro's government.

"We urge those who support this regime, from everyday employees getting by on food subsidies to the Venezuelan security forces sworn to support the constitution, to stop enabling repression and corruption and to work with the National Assembly and its duly elected leader, Juan Guaido, in accordance with your constitution on a peaceful return to democracy," Pompeo said in a statement.

"The Venezuelan people and the international community will remember and judge your actions. Now is the time to convince the Maduro dictatorship that the moment has arrived for democracy to return to Venezuela," he said.

"The Secretary reinforced the U.S. commitment to the National Assembly, the only legitimate and last democratically elected institution in Venezuela, and the re-establishment of democracy in Venezuela," spokesperson Robert Palladino added.

Maduro was sworn in for a second six-year term under a cloud of skyrocketing inflation, shortages of basic food and medicine and an exodus of Venezuelans to neighboring countries, following an election that was boycotted by the opposition.

The Lima Group -- a bloc of 14 Latin American powers and Canada -- urged Maduro to renounce his term and hand over power to parliament, although Mexico's new leftist government stayed neutral.

The European Union has also said that last year's election was marred by fraud and has extended sanctions.

The United States has for years put pressure on Maduro and his predecessor Hugo Chavez, with President Donald Trump publicly musing about military intervention.

On Tuesday, the United States imposed its latest sanctions as it targeted seven Venezuelans allegedly involved in black-market currency exchanges that generated billions of dollars in illicit profits.